There are numerous applications in Industrial Process Control that require the measurement of the conductivity of a water-based solution. One such application is disclosed in U.S. Pat. No. 4,733,798, which is herein expressly incorporated by reference. This type of system is related to institutional ware washing and dishwashing machines, and the like. In such systems, automatic detergent dispensers are employed, for dispensing additional detergent into the wash water during a wash cycle, upon detection that the detergent level in the water has fallen below a desired value.
The conductivity of a water-based solution is often a direct analog of the concentration of dissolved chemicals in the solution. In cases where the concentration of certain chemicals in the solution is critical to successful operation, continuous real-time measurement and control is required. Conductive probes with exposed electrodes are the most common method for measuring conductivity. They are inexpensive and the interface electronics are simple, but all suffer from inherent contamination and corrosion of the exposed conductive electrodes. More caustic chemicals cause more rapid degradation. Corrosion and plating of the conductive probe electrodes cause significant measurement errors and necessitate frequent servicing to clean or replace the probe assembly.
Inductive Probe systems have been created to address the inherent problems with conductive probes but their much greater cost has seriously limited their applications, especially with traditionally low-cost chemical control systems.
What is needed, therefore, is a system for emulating conductive probe systems which is inexpensive and permits the elimination of conductive probes in new chemical control systems, and the replacement of conductive probes in existing installed systems.